Here's a new phrase to conjure with: the 'motherhood penalty'. It is being advanced by the Fawcett Society, which has campaigned for women's rights since 1866.

The 'motherhood penalty' refers to the fact that a woman's income often declines when she becomes a mother. Men and women may be at level pegging in the job market when they are young and childless. They compete with a reasonable degree of equality for promotion and advancement.

But then – ping! When a baby arrives, a woman becomes less competitive, less likely to be promoted, and she is earning less than her male cohorts.

For every child she bears, it is calculated that a woman loses 13% of her earnings. That's the 'motherhood penalty'. And the Fawcett Society is resolved to eliminate it from all areas of employment, public and private, large and small.

Yet, I have news for the Fawcett campaigners: it's a lifetime condition.

There's a 'motherhood penalty' when a beloved child wrestles with painful situations, be it depression, mental illness, alcoholism or suicide. It's the mothers sitting up late at night, worrying; the mothers lighting candles and praying for succour and moving heaven and earth to help an afflicted son or daughter.

Go to any group counselling session for families with problems and you will be sitting in a circle with mothers desperate to rescue adult offspring who have gone off the rails.

That's a 'motherhood penalty', too: the lifelong sense of anxiety for a child brought into the world. I don't say that fathers don't have such feelings too. They do. But there are plenty of studies which show that men, for whatever reasons of brain-wiring, are better able to compartmentalise their thinking and concerns.

Almost every woman I know of my own vintage – most of them now grandmothers – still has sleepless nights over their adult children: the son who has gone on a motorbike ride through the Rocky Mountains; the daughter who has just had a bruising relationship break-up (and the girl is now middle-aged); the son who has come back home to live because of an impending divorce, or who has lost his job, or is drinking too much; the beloved only child who has decided to emigrate to faraway New Zealand?

Yes, the 'motherhood penalty' is an apt phrase, indeed, for a condition which will last a lot longer than questions about whether your pay packet is keeping up with the guys.

I wouldn't want to disparage the Fawcett Society's campaign to support equal pay for mothers; they are only trying to be helpful and encouraging to women with children. And that's a good thing to do. Sheryl Sandberg sought to do likewise in her book Lean In.

We hear plenty about "unwanted pregnancies", but rather less about pregnancies which would be dearly wanted if the circumstances were supportive.

But I do believe that some of the issues arising from the 'motherhood penalty' come not just from social structures around either career organisation or child-care, but from Nature itself.

It's been well established that as soon as a woman has children, she drives more prudently – I'm talking about averages, not the odd female petrol-head, addicted to speed. I remember watching a niece by marriage strap her young children into car safety seats with such attention to detail she checked every element of the apparatus before setting off behind the wheel. There's a mother's protective care, I thought. It comes directly from her instinct.

Even in my own case – reckless and feckless though I was by temperament – once I had children, I lost a certain edge for risk. As a journalist, I turned down foreign assignments that would take me away from home base for too long, or might be more dangerous. I cannot say that any employer was discriminatory towards me: the 'motherhood penalty' came from nature's own promptings, not any patriarchal system. I became less tough in some ways, and certain scenes brought me more easily to tears.

Remember Mary Robinson when she visited a famine-stricken Ethiopia – this self-assured lawyer in floods of tears at the sight of starving children? That's another side of the 'motherhood penalty': it prompts pity and compassion.

Even Margaret Thatcher, who kept her Cabinet in a state of apprehensive submission, wept openly when Mark, the son who seemed to embody the spoilt-brat syndrome, was lost in an African desert. That, too, was the 'motherhood penalty'.

It is true that many women, overall, lose income and job promotion, which is a key to income, when they become mothers. And it is evident that mothers worry about their children all their lives – "until you go down into the grave" as I was once told by an older mother.

But in the face of the responsibilities and 'penalties' of motherhood, what is miraculous is the number of women who will move any mountain, go to any length and pay any price, to become mothers. The women having four cycles of IVF, the women imploring overseas orphanages to adopt an abandoned child, the women who will agree to a dodgy deal on surrogacy.